python3: 'where' keyword

[Nick Coghlan]

Paul Rubin wrote:
> Nick Coghlan <ncoghlan at iinet.net.au> writes:
> 
>>I think having to keep the names unique within the statement you are
>>currently writing is a reasonable request :)
> 
> 
> Um, you could say the same thing about the function, the module, etc. ;)
> 

And, indeed, that is what Python currently says. When writing code, the relevant 
namespaces are the builtins, the module, any containing functions and the 
current function. Inadvertent conflicts with any of those can have surprising 
side effects.

The idea of 'where' is to push that down one level, and allow a namespace to be 
associated with a single statement.

Trying to push it a level further (down to expressions) would, IMO, be a lot of 
effort for something which would hurt readability a lot.

Compare:

   x = sqrt(a) + sqrt(b) where:
     a = 2.0
     b = 3.0

This brings the operation we care about (add the sqrt's of 2 and 3) right up 
front. A folding code editor could actually hide the details quite easily. We 
can look inside the statement if we want to know what x & y actually are.

Versus:
    x = (sqrt(a) where:
          a = 2.) \
        + sqrt (a) where:
            a = 3.

We haven't gotten rid of anything here - all the stuff we're interested in 
clearing out of the way is still embedded in the middle of our statement.

Also not insignificantly, we're trying to put a suite inside an expression, 
which will be rejected for all the reasons that have kept lambda restricted to a 
single expression despite numerous complaints over time.

Now, nothing in the idea of a statement local namespace actually *rules out* the 
prospect of an expression local namespace, so it could be added at a later date. 
However, doing so would require some actual use cases, and an 
expression-friendly syntax. Perhaps something that involves providing the 
namespace directly, like:

   x = (sqrt(a) where (a=2.0)) + (sqrt(b) where (a=3.0))

It seems to make more sense to try for statement local namespaces *first*, and 
then see if expression local namespaces are worth it.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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